Does Red Bull Sugarfree break a fast?

No. It has about 10 calories and no sugar, so it keeps you fasted for weight loss and ketosis. It still has 80 mg of caffeine.

Does Monster Zero Ultra break a fast?

Not for most goals. It has no sugar and very few calories. The caffeine is the main thing to watch.

Do energy drinks break autophagy?

Regular ones do, because of the sugar. Sugar-free cans probably do not, but no study has tested them directly.

Are energy drinks safe while fasting?

Healthy adults usually tolerate one sugar-free can. People with heart rhythm problems, high blood pressure, anxiety or a pregnancy should avoid them.

Do Energy Drinks Break a Fast? It Depends

Reviewed by the FastTrack team · Updated June 2026

Short answer: It depends

It depends on the can. Regular energy drinks are sugar drinks: an 8.4 oz Red Bull has 26 g of sugar and 110 calories, which breaks a fast. Sugar-free versions such as Red Bull Sugarfree or Monster Zero Ultra have about 0 to 10 calories and keep most fasts intact.

Calories	~110-230 kcal (regular), ~0-10 kcal (sugar-free)
Breaks a weight-loss fast?	Partial
Breaks ketosis?	Partial
Breaks autophagy?	Partial
Insulin impact	High for regular, minimal for sugar-free

Why regular energy drinks break your fast

The sugar in a regular can is absorbed quickly and raises blood sugar and insulin within minutes, which ends the fasted state and pauses ketosis. A 16 oz can of regular Monster carries more than 50 g of sugar, more than many people eat at breakfast.

Sugar-free cans and caffeine

Sugar-free energy drinks use sucralose, acesulfame potassium or erythritol and stay under about 10 calories, so they do not break a fast for weight loss or ketosis. The concern moves to caffeine: most cans hold 80 to 300 mg, and on an empty stomach that can mean jitters, palpitations or nausea. Check the caffeine dose on the label.

How much is safe

If you drink one, choose sugar-free, have it in the morning and keep it to one can. Count it toward a daily caffeine total under 400 mg.

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Frequently asked questions

Does Red Bull Sugarfree break a fast?: No. It has about 10 calories and no sugar, so it keeps you fasted for weight loss and ketosis. It still has 80 mg of caffeine.
Does Monster Zero Ultra break a fast?: Not for most goals. It has no sugar and very few calories. The caffeine is the main thing to watch.
Do energy drinks break autophagy?: Regular ones do, because of the sugar. Sugar-free cans probably do not, but no study has tested them directly.
Are energy drinks safe while fasting?: Healthy adults usually tolerate one sugar-free can. People with heart rhythm problems, high blood pressure, anxiety or a pregnancy should avoid them.